1. Field of the Invention
The present invention relates to a system and method for attaching a payload-carrying gondola or car to a hull of non-rigid airships.
2. Description of Related Art
Airships remain a viable and relatively inexpensive means for performing a variety of useful functions. While currently not universally accepted as a mass transit means, airships do have the ability to accommodate passengers and are extremely useful for advertising purposes (e.g., sporting events) as well as for both military and civilian surveillance applications. In particular, certain airships have the ability to carry large antenna arrays within their hulls or envelopes (hereafter "envelopes"). This, coupled with the airship's ability to remain on station for days at a time (i.e., stay over the same area) makes an airship ideal for combat type surveillance applications. Further, airships have a relatively low radar signature, making them difficult to detect. Civilian surveillance applications include border patrol, drug smuggling operations, and other criminal applications.
One of the most critical features of an airship is the payload-carrying gondola or car (hereafter "car"). Of the three most common types of airships, rigid, semi-rigid, and non-rigid or blimp, the non-rigid airships provide the biggest challenge in the area of payload attachment. Unlike rigid and semi-rigid airships which have at least some type of an internal skeleton to which the payload might be attached, the non-rigid airships rely solely on an envelope and its gas contents for shape and structure. The payloads may be comprised of any or all of the following, including a cockpit, an engine, persons, and various other cargo or equipment.
For larger volume airships (e.g., greater than 5000 cubic meters), the prior art teaches the use of an internal catenary system as shown in prior art FIG. 3. For ease of explanation, the envelope 30 has been divided by line A into upper and lower portions. The internal catenary system is basically an internal cable suspension system wherein two or four longitudinal load curtains 32 (only one shown) are attached to the internal upper surface 34 of an envelope 30. Attached to the lower edges of the load curtains 32 and extending downward through gas tight seals (not shown) in the lower envelope surface 36, is a cable system using either steel or nonmetallic Aramid.TM. cables 38. These cables 38 are tensioned and attached to strategic hard points 40 on the car structure 42. The load curtains 32 and cables 38 are geometrically designed so that the weight of the car payload is transferred by the suspension cables 38 to the envelope curtains 32 and is spread onto the envelope's upper surface 34 through the curtains 32.
The conventional internal catenary system is disadvantageous for a number of reasons. First, the conventional system requires several holes in the lower envelope skin to allow the suspension cables to pass through. These holes weaken the envelope's integrity and are possible sources of gas leaks. Second, the alignment of the suspension cables from the load curtains through the lower skin to their attachment points in the gondola is critical. Any misalignment is corrected by fitting cable fairleads which lead to high cable side-loads and possibly additional loads on the envelope. The ultimate cable life is severely reduced when a fairlead is used to correct a misalignment problem. Third, the internal cable system consists of a complex assembly of cables and pulleys which are installed inside the envelope prior to inflation with the lifting gas. This assembly is difficult to maintain and repair once the envelope is inflated. Fourth, the attachment of the gondola to the envelope is a complex process and requires suitable equipment to tension the catenary cables during the airship assembly. Finally, all of the gondola suspension loads are transferred through the upper load curtain/envelope interface. The various envelope breaches and excessive load concentrations inherent to conventional internal catenary systems place severe limitations on airship and payload size.
For smaller airships, a conventional external catenary system is commonly used and employs a suspension system external to the envelope 40 as shown in FIG. 4. Cables 42 are attached tangentially to load patches 44 bonded to the outside envelope surface 46. The cables are tensioned and attached to the lower edges of the car/gondola 48. This method does not require any complex internal load curtain or suspension cable system and the pressure envelope integrity is not breached by cable suspension gaiters and load rings. But the conventional external catenary system has disadvantages of its own.
First and most importantly, the success of the conventional external catenary system has been limited to relatively small volume airships, carrying small payloads. Second, the external cables detract from the aesthetics of the airship, increase drag and can hinder ground crew personnel and passengers. For example, depending on where the external cables attach to the car, it may be difficult to access the cockpit or engine compartments for maintenance. Further, the cables might obstruct the view of passengers, limiting passenger enjoyment and, consequently, purchasing incentives.
A final conventional suspension system is the aerostat system which is used mainly for aerostats and airships with small payloads. The aerostat system employs a system similar to the external catenary, however, with these small payload applications the car is attached directly to the external skin of the envelope without additional cables. In this method the car or payload is attached to a number of fabric tabs bonded to the lower skin of the envelope. While this system is simplistic in its design and therefore advantageous in some situations, the system is severely limited by the payload that can be carried by the airship. Heavy payloads would produce large distortions in envelope shape, giving rise to local areas of high fabric stress.